Negotiated Study One: Further System Research: The Kidneys and The Brain and Central nervous System

Kidneys: The Filtration Units

• The urinary system is concerned with the formation and elimination of urine
• In an adult, more than 2500 pints of blood pass through the kidneys each day
• Blood enters via the renal arteries and is filtered to remove most of the waste products of metabolism.
• Seven pints of filtration are produced every hour. Purified blood returns to the body circulation via the renal veins
• The filtering process is carried out by more than two million tiny kidney units, or nephrons, which produce a highly concentrated solution of chemicals known as urine, which is harmful to the body if allowed to remain.
• Urine flows from the nephrons, first into the funnel shaped renal pelvis and then into the ureter. 
• Waves of muscular contraction passing down the ureters push the urine into the bladder.
• With continuous filling, the bladder, a muscular bag, expands until it holds about one pint of fluid. 
• A circular band of muscle around the neck of the bladder, the sphincter, controls the release of urine from the body.

• Themost important route of excretion is through the kidneys - the paired filtration units lying at the back of the abdomen. 
• These produce a constantly adjusted trickle of urine which is voided when the bladder becomes uncomfortably full.
• Urine, a solution in water of urea, salts and other soluble wastes, is produced by the kidneys from the blood that enters them, under high pressure, through the renal arteries.
• The renal arteries are large side branches of the aorta and together receive about a quarter of the volume of blood pumped out by the heart at each beat.
• All of the blood in our bodies passes through the kidneys about twenty times every hour, but only one fifth of the plasma is filtered into the kidneys tubules.
• Each kidney is roughly the size of a child's fist
• Seen cut open it has an outer pale layer called the cortex, and a darker core, densely packed with blood vessels that divide and subdivide, called the medulla
• On the medial aspect of each kidney lies a urine collecting cavity, the renal pelvis, leading down through a thin walled tube, the ureter, into the distensible bladder
• Each kidney has about one million separate filter units, called nephrons. A nephron consists of a cup shaped structure, Bowmans capsule, which encloses a know of capillaries, known as glomerules that carries blood from the renal artery, and a renal tubule.
• Each tubule is between one and two inches long, and loops, rather like a trombone, in and out from the cortex, where the capsule lies into the medulla
• The tubules unite into collecting ducts, and the ducts empty their contents into the renal pelvis.

• As blood under pressure passes through each glomerus, water, dissolved salts, sugar, urea and many other small molecules are forced through the walls of the capillaries and into the capsule.
• The blood, now more concentrated because it has lost about one fifth of its plasma, passes from the glomerus into a network of fine vessels surrounding the loops of each tubule, and will eventually rjoin the general circulation through the renal vein.
• Before it does however, another important process must be carried out.
• Although the kidney is described as excreting water, it has in fact a duty to conserve water.
• our bodies need water as a medium in which every biochemical process can take place . Only if we were water logged could water be said to be a waster material with a nuisance value.
• It is simple that a solvent is needed for true wastes, to remove them from the body, that large amounts of water pass into the kidney tubules
• Nearly all the water i, therefore, recovered from the filtrate in its journey through the loops of the renal tubule, passing back into the bloodstream through the tubule walls into the networks of blood vessels.

• At the same time by process of chemical transfer through the tubule walls, sugar and other materials that the body needs, but which have passed into the filtrate because of the small size of their molecules, are also conserved by reabsorption into the blood.
• So efficient is the reabsorption of sugar, for example, that not a trace is normally detectable in the urine. 
• After passing down the ureters, the urine is stored in the bladder, to be discharged through the urethra to the outside when convenient.

The Brain and Central Nervous System

• Monitoring and controlling every aspect of the body's activities, no matter whether we are awake or asleep, is the vast and complex communications network called the nervous system.
• At the very heart of the network is the central nervous system, consisting of the brain and spinal cord. 
• And weaving through and penetrating to every near and outlying region of the body are the almost countless branches of the peripheral nervous system.
• The all important brain, a densely packed mass of nervous tissue weighing nearly three pounds, and the spinal cord, continuous with the brain.
• Acting as channels of communication between the central nervous system and the rest of the body are the afferent or sensory receptors, and the efferent, or motor, nerves, which carry outgoing messages to effector structures such as the voluntary muscles of the limbs.
• Sensory and motor nerves of the peripheral system leaves the spinal cord separately, between the vertebrae, but then unite to form thirty one pairs of spinal nerves, in which there are both sensory and motor nerve firbres
• Branches of the spinal nerves spread out to reach all parts of the body surface and all skeletal muscles

• Leaving the brain directly are twelve pairs of cranial nerves, each passing through the separate aperture in the skull
• There are more than ten thousand million nerve units, or neurons, in the brain, but even these account for only one tenth of the brain cells. 
• The remainder are surrounding and supporting cells called the neuroglia or just glia.
• Together the neurons and glia make soft, jellylike tissue; without the support of the surrounding skull it would distort and sag.